Currently available portable scales now teach a dial graduated to indicate desired weight units, a rotatable pointer adapted to cooperate with said dial and spring controlled rack and gear means for actuating the pointer. These devices may also contain main load springs arranged to accept the load of heavier weights and to communicate with the spring controlled rack and gear means.
It is generally known that the relationship between the force and displacement of a coil spring is non-linear in the early portion of the curve illustrating the relationship between force and displacement for a given spring. It is also known that within this non-linear portion of the curve, an increasing amount of force is necessary to displace the spring each additional unit of distance. This phenomenon normally results in an inaccurate weight measurement.
This problem has been addressed in prior art devices by providing a secondary spring, in addition to the main loading spring, which works against the main loading spring in the early portion of the curve by exerting an initial force on the main spring. See for example, Clarke, U.S. Pat. No. 2,612,042, disclosing a force measuring device. This patent discloses a measuring device that includes two main springs for absorbing the load and an additional secondary spring working against the main springs to compensate for manufacturing errors in the main springs and to minimize the inaccuracy illustrated by the non-linear portion of the force vs. displacement curve. While the use of a secondary spring places the operating range of this device outside the lower non-linear part of the force vs. displacement curve, the additional spring adds to the product's cost and increases the anchoring problems of the device. Furthermore, because of these and other errors associated with a plurality of loading springs, the resolution of this device is sacrificed, e.g. Clarke shows a resolution of only one part in 100.
In addition to the above, Clarke teaches that the pre-load spring gradient can be varied through the use of an adjusting abutment and nut. The addition of this added mechanism, while permitting corrections for errors inherent in the springs and errors caused by temperature conditions, causes inaccuracies associated with additional moving parts that wear with continued use.
Prior art devices, like Clarke's scale, usually require spring bearings, thin steel members having spiral cut out portions therein, or side restraints to position the main load springs. While these elements keep the springs from moving laterally, they too add to the overall inaccuracy of the device through "dead band" effects, frictional errors created by the moving parts of the scale that create errors in the readings.
Accordingly, there is still a need for a spring weighing device that minimizes friction and dead-band errors in reading weight measurements. There is also a need for a spring scale having greater resolution that permits adjustment for peripherals and tare with no secondary effect on the spring gradient or the span of the instrument.